Wales and south-east England rents increased the most last month. Photograph: John James/Alamy

A record rise in rent in August has left thousands of tenants struggling to pay their landlords, figures show.

Rents rose by their fastest rate in a year last month, with the average tenant now paying £713 a month, according to LSL Property Services, owners of the UK's largest lettings agent network.

Its figures, based on an analysis of 18,000 UK properties, show that the average rent in England and Wales rose by 2.1% in August, adding £27 a month to the rent tenants were paying in August last year.

Rents in Wales and the south-east increased the most last month, but over the year London rents have risen the most. A typical tenant in the capital is now paying £1,025 a month, which is 6.6%, or £63 a month, more than they were paying this time last year. Only the Yorkshire and the Humber region has been home to falling rental prices, where tenants' rent has come down by 0.5% in a year.

"We are in the thick of the busiest time of year for the rental market, and red hot demand for properties is driving rents up at their fastest monthly pace in the last 12 months," said David Newnes, estate agency managing director of LSL Property Services.

"Recent graduates moving for their first jobs have further exaggerated the long-term and growing demand from frustrated buyers."

The figures also show that rocketing rents are having a serious impact on tenants' ability to pay. Last month, tenant arrears increased for the first time since April, with almost 11% of all UK rent unpaid or late by the end of the month, compared with 9% in July. Unpaid rent totalled £300m across the UK in August.

"With rents rising so quickly, soaring inflation and an uncertain economic outlook, over the long-term we anticipate that rental arrears will become a growing financial problem for landlords," said Newnes.

Separate figures from tenant referencing agency HomeLet, based on data from 3,000 letting agents, puts London rent rises even higher, reaching an average of £1,202 in August. It claims rents in the capital have increased by a record 12.2% in the past year, while the average salary in the region has risen by 2.4% over the same period.